Detecting & Fixing Memory Leaks in Java (Spring Boot) — A Practical Guide

🧠 Introduction

Memory leaks are one of the most dangerous performance issues in Java applications. They silently consume heap memory, degrade performance, and eventually crash production systems.

Even though Java uses Garbage Collection, memory leaks still happen when objects remain referenced and cannot be cleaned up.

In this guide, you will learn:

✔ What memory leaks are
✔ How to detect them
✔ Tools professionals use
✔ How to fix them
✔ A working Spring Boot demo example

🚨 What is a Memory Leak?

A memory leak occurs when:

Objects are no longer needed but are still referenced, preventing Garbage Collection.

Over time, heap usage grows until the application throws:

java.lang.OutOfMemoryError: Java heap space

🔥 Common Causes of Memory Leaks
1️⃣ Static Collections Holding Objects
public class CacheStore {
public static List cache = new ArrayList<>();
}

❌ Objects never released
✔ Heap memory keeps growing

2️⃣ Unclosed Resources
Connection conn = dataSource.getConnection();

❌ Connection not closed → resource leak

✔ Use try-with-resources:

try(Connection conn = dataSource.getConnection()) {
// use connection
}

3️⃣ ThreadLocal Misuse

Improper ThreadLocal cleanup can leak memory in thread pools.

4️⃣ Listener & Callback References

Listeners registered but never removed remain in memory.

🔬 How to Detect Memory Leeks
✅ Step 1: Monitor Heap Usage

Use:

✔ JVisualVM
✔ JConsole
✔ Micrometer + Prometheus
✔ Grafana dashboards

If heap memory keeps growing → leak suspected.

✅ Step 2: Generate Heap Dump

Run:

jmap -dump:live,format=b,file=heap.hprof

✅ Step 3: Analyze Heap Dump

Use:

🛠 Eclipse Memory Analyzer Tool (MAT)

Open heap dump → check:

✔ Dominator Tree
✔ Leak Suspects Report
✔ Objects retaining memory

🧪 Spring Boot Memory Leak Demo
Leak Code Example

@RestController
public class LeakController {

    private static final List<byte[]> memoryLeak = new ArrayList<>();

    @GetMapping("/leak")
    public String leak() {
        memoryLeak.add(new byte[1024 * 1024]); // 1MB
        return "Added";
    }
}

Every call adds 1MB → memory grows continuously.

📊 Memory Leak Behavior
BEFORE FIX

✔ Heap usage continuously increases
✔ GC unable to reclaim memory
✔ Application slows down

AFTER FIX

✔ Memory stabilizes
✔ GC reclaims unused objects
✔ Performance improves

🛠 Fixing the Leak
❌ Problem

Unbounded static list storing objects.

✅ Solution Options

✔ Limit cache size
✔ Remove unused objects
✔ Use WeakReference
✔ Use caching libraries (Caffeine, EhCache)

Example fix:

private static final List<byte[]> memoryLeak = new ArrayList<>();

@GetMapping("/fix")
public String fix() {
    if(memoryLeak.size() > 10) {
        memoryLeak.clear();
    }
    memoryLeak.add(new byte[1024 * 1024]);
    return "Managed";
}

📈 Production Monitoring Best Practices

✔ Enable GC logging
✔ Track heap usage metrics
✔ Set memory alerts
✔ Run periodic heap analysis
✔ Load test before release

🧰 Tools Professionals Use

✔ VisualVM
✔ Eclipse Memory Analyzer Tool
✔ JProfiler
✔ YourKit

🎯 Key Takeaways

✅ Garbage Collection does NOT prevent memory leaks
✅ Static references are common leak sources
✅ Heap dump analysis is essential
✅ Monitoring prevents production failures

🔗 GitHub Demo Repository

👉 Add your GitHub link here

Example:https://github.com/yourusername/spring-boot-memory-leak-demo

🚀 Conclusion

Memory leaks are silent killers of application performance. By monitoring memory usage, analyzing heap dumps, and following best coding practices, you can prevent crashes and ensure system stability.